ver, it is quite
unnecessary if the directions already given are followed.

The student's subsequent success will so largely depend upon his
acquaintance with the resources of his blow-pipe, and on the facility
with which he can take advantage of them, that no pains should be spared
in the effort to become expert in its management as soon as possible. A
few experiments should now be made, therefore, upon the adjustment of
the flame, until the student is able to produce and modify any form of
flame with promptness and certainty.

[Illustration: FIG. 5.]

The remaining apparatus used in glass-working consists of triangular and
other files, charcoal pastils for cutting glass, pieces of sound
charcoal of various diameters with conical ends; it is convenient to
have one end somewhat less pointed than the other (Fig. 5). Corks of
various sizes; the smallest, which are most frequently needed, should be
carefully cut with sharpened cork borers from larger corks. Besides
these there should be provided some freshly distilled turpentine in
which camphor has been dissolved,[1] fine and coarse emery powder, and
some sheets of cotton-wadding, an india-rubber blowing-bottle, glass
tubes, a little white enamel, and a pair of iron tongs.

[1] Half an ounce of camphor to about six ounces of turpentine will do
very well.

CHAPTER II.

_VARIETIES OF GLASS AND THEIR MANAGEMENT._

All the varieties of glass that are ordinarily met with contain silica
(SiO_{2}) associated with metallic oxides. In a true glass there are at
least two metallic oxides. The unmixed silicates are not suitable for
the purposes of glass. They are not so capable of developing the viscous
condition when heated as mixtures--some of them are easily attacked by
water, and many of those which are insoluble are comparatively
infusible. There is generally excess of silica in glass, that is, more
than is necessary to form normal silicates of the metals present. The
best proportions of the various constituents have been ascertained by
glass-makers, after long experience; but the relation of these
proportions to each other, from a chemical point of view, is not easy to
make out.

The varieties of glass from which tubes for chemical glass-blowing are
made may be placed under three heads, and are known as[2]--

Soft soda glass. Also known as French glass.
Lead glass. Also known as English glass.
Hard glass.

[2] For details of the composition